Method and Apparatus for Sharp Bending High Strength Panels

ABSTRACT

A method and apparatus for forming a flange on a panel. The flange is formed under compression to provide a reduced outside radius on the finished flange. The flange may be formed about an outside corner with a distal edge of the flange being restricted by a limiting edge formed on a die. Alternatively, the flange may be formed into an inside corner of a die with a compressive force being exerted on a distal edge of the flange by a compressing tool while a forming tool forms the flange with a reduced final outside radius.

TECHNICAL FIELD

This disclosure relates to bending high strength sheet metal panels that are characterized by low formability.

BACKGROUND

The radius of a bend in traditional sheet metal bending processes, for example in flanging and hemming operations, is limited by the ability of the material to elongate before fracturing. This parameter is defined in a tensile test as the total elongation of the material. External layers of the sheet metal undergo the most extensive elongation in bending processes. The external layers are supported by the inner layers that experience substantially less elongation.

The general formula for maximum strain in sheet metal bending operations may be expressed as:

e=t/(2R _(mid) surface)=Total Elongation (t=sheet thickness)

From this equation, the minimum bending radius

R _(mid)surface=t/(2 Total Elongation)

For Total Elongation=0.25, which is typical for aluminum alloys,

R _(mid)surface=t/(2×0.25)=2t

The inner surface corresponding to the radius of the tool can be calculated as:

R _(inner)surface=1.5t

In practice, the typical inner radius for a lmm thick Aluminum Alloy 6111-T4 sheet is limited to a minimum of 2.5t to avoid splits on the outer radius of a flange or hem in production.

A number of attempts have been made to form sharper radii when bending, flanging or hemming. For example, U.S. Pat. No. 6,865,917 discloses bending material using an elastic tool. The elastic tool undergoes severe compression and creates elevated hydrostatic pressure on the external surface of the blank that undergoes the most significant stretching. Higher elongation can be obtained when hydrostatic pressure is applied for many materials.

Another approach, disclosed in U.S. Pat. No. 6,928,848, is to pre-bend the sheet to a larger radius where a sharp flanging radius is desired and then perform a cam flanging operation to form a much sharper inner bending radius on the sheet metal blank. For example an AA6111-T4 aluminum panel was pre-bent to approximately 2.5t, followed by a cam flanging operation to form an inner diameter of R_(inner)=0.5t or less. Bending the panel to this extent is possible because of the material flow provided to the stretched area from the flange of the blank.

Another example of a problem relating to sharp bending ultra-high strength steel thick sheets of material, for example, a 6 mm thick sheet of material that has a maximum elongation of approximately 12% exhibited reduced elongation. The reduced elongation characteristic translates into the minimum radius of the mid surface as being:

R _(mid)surface=t/(2×0.12)=4t

A minimum mid surface radius of 4t on a 6 mm thick sheet is equal to 24 mm, while the outer radius is 30 mm. An outer radius of 30 mm does not appear to be sharp and is not desirable.

The above problems and other problems are addressed by this disclosure as summarized below.

SUMMARY

According to the proposed solution, there are two concepts relating to approaches for creating a compression zone in the area of potential stretching and splitting. The first concept is to pre-bend the blank to a generous radius, for example of 6t-7t, and then applying a substantial axial compression force to extrude the material into a sharp inside corner. Compressing the flange creates a compression stress in the area where substantial elongation occurs in a conventional bending process. The second concept is to bend the blank at an angle having a large bending radius until the edge of the blank is received in a limiting edge of the die causing the flange to form an arc-shaped area, and then compressing the arc-shaped area of the flange into the corner.

According to one aspect of this disclosure, a method of forming a sharp bend on a panel comprising pre-bending the panel to form a flange having a radius at a base portion of the flange between the flange and a body portion of the panel. The flange is clamped between a distal end of the flange and the base portion with a clearance being defined between the flange and an anvil surface. The flange is then compressed against the anvil surface.

According to other aspects of the method, the method may further comprise clamping the body portion of the panel between a clamping pad and a die while a punch is reciprocally moved to perform the pre-bending step and then performs the compressing step. The method may be further characterized by providing the die with a limiting edge that is engaged by the distal end of the flange during the compressing step. The distal end of the flange is formed into engagement with the anvil surface of the die by the punch. The distal end of the flange slides along the anvil surface until the distal end engages the limiting edge.

The clearance defined between the flange and an anvil surface may be an arc-shaped area that defines a clearance adjacent the base portion of the flange. The method may further comprise compressing the arc-shaped area against the anvil surface causing the base portion of the panel to flow into the base portion of the flange.

According to another aspect of this disclosure, an alternative method may comprise clamping the body portion of the panel between a clamping pad and a die. The distal end of the flange is compressed between a compressing tool and the die. The base portion of the flange is formed into a corner of the die with a forming tool while the flange is compressed by the compressing tool. The clearance defined between the flange and the corner of the die is adjacent the base portion of the flange. The base portion of the flange is forced to flow into the corner of the die.

According to another aspect of this disclosure, a tool for forming a flange on a panel is provided. The tool may include a die having a limiting edge engaging a distal end of the flange, a clamping pad for clamping the panel against the die, and a punch that initially engages the panel to form the flange with a radius at a base portion of the flange. The punch forms the flange against an anvil surface of the die while the flange is compressed between the clamping pad and the limiting edge to reduce the radius of the flange.

According to other aspects of the tool, an outside corner may be provided on the die and the punch may engage the panel to initially form the flange around the outside corner. The clamping pad clamps the panel on one side of the outside corner. The base portion of the flange may have an arc-shaped portion that defines an arc-shaped clearance relative to the anvil surface. The punch forms the arc-shaped portion of the flange into the base portion of the flange.

According to an alternative embodiment, a tool for forming a flange on a panel is provided that includes a die having an inside corner that receives a base portion of the flange in a spaced relationship. The base portion of the flange has an initial radius. A clamping pad is provided for clamping the panel against the die. A compressing tool engages a distal edge of the flange. A forming tool forms the base portion of the flange into the inside corner while the flange is compressed by the compressing tool to form a final radius that is smaller than the initial radius.

According to other aspect of the alternative embodiment of the tool, the inside corner may have a radius that corresponds to a final outside radius of the flange and the forming tool may have a radius that corresponds to a final inside radius of the forming tool. The compressing tool may move with the forming tool to compress the flange while the forming tool forms the final radius. The flange may be pre-formed in a forming die to a radius that is greater than a minimum radius at which a fracture may occur from bending the flange.

The above aspects of this disclosure and other aspects will be better understood in view of the attached drawings and the following detailed description of the illustrated embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view of a sheet metal panel in a die set in position to be bent to form a flange;

FIG. 2 is a diagrammatic cross-sectional view of a sheet metal panel in a die set in the process of being pre-bent to form a large radius bend;

FIG. 3 is a diagrammatic cross-sectional view of a sheet metal panel in a die set in the process of compressing the large radius bend formed as illustrated in FIG. 2;

FIG. 4 is a diagrammatic cross-sectional view of a sheet metal panel in a die set that has a sharp bend after being compressed as illustrated in FIG. 3;

FIG. 5 is a diagrammatic cross-sectional view of a sheet metal panel in an alternative embodiment of a die set in the process of being pre-bent to form a large radius bend; and

FIG. 6 is a diagrammatic cross-sectional view of a sheet metal panel after being compressed to form a sharp bend in the alternative embodiment of a die set illustrated in FIG. 5.

DETAILED DESCRIPTION

A detailed description of the illustrated embodiments of the present invention is provided below. The disclosed embodiments are examples of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale. Some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed in this application are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art how to practice the invention.

Referring to FIG. 1, a tool 10 operating on a panel 12 is illustrated prior to beginning to form the panel 12. The tool 10 includes a die 14 upon which the panel 12 is supported. A clamping pad 16 clamps the panel 12 to the die 14. A punch 18 engages the panel 12 to form the panel as will be described below with reference to FIGS. 2-4.

Referring to FIG. 2, the tool 10 is shown forming a flange 20 on the panel 12. The panel 12 is clamped between the clamping pad 16 and die 14. The punch 18 forms the flange 20. A distal edge 22 of the flange 20 is shown as it is formed toward a limiting edge 24 of the die 14. A base portion 26 of the flange 20 has an initial outside radius 28 adjacent the base portion 26. The initial outside radius 28 is greater than a minimum radius in which a fracture may occur using conventional bending techniques. An anvil portion 30 is provided on the die 14. The punch 18 forms the flange 20 against the anvil portion 30 and around an outside corner 32 of the die 14. The outside corner 32 is formed at the intersection of a clamping side 36 and a forming side 38 of the die 14. A clearance 42 is defined between the arc-shaped portion 40 of the flange 20 and the forming side 38 of the die 14.

Referring to FIG. 3, the tool 10 is shown with the clamping pad 16 holding the panel 12 against the die 14. The punch 18 is shown forming the flange 20 against the anvil portion 30 of the die 14. The distal edge 22 of the flange 20 is shown engaging the limiting edge 24 of the die 14. The arc-shaped portion 40 is shown as it is compressed against the anvil portion 30 with material from the arc-shaped portion 40 being compressed providing material flow towards the sharp corner 32. The clearance, shown in FIG. 3, is reduced compared to the clearance provided in FIG. 2. The limiting edge 24 of the die 14 prevents movement of the distal edge 22 of the flange 20. The flange 20 is compressed by the punch 18 between the base portion 26 and the limiting edge 24.

Referring to FIG. 4, the tool 10 is shown in its final position with the panel 12 having a flange 20 formed against the die 14. The punch 18 is shown with the flange 20 completely formed about the outside corner 32 between the clamping side 36 and forming side 38 of the die 14. The flange 20 has a final outside radius 44 formed outboard of the outside corner 32. The final outside radius 44 of the flange 20 has a final radius that is tighter than radiuses that may be achieved by conventional forming techniques on similar materials.

The suggested sharp bending processes are based on the idea of creating compression in the areas of substantial elongation in conventional processes. The first version of the process is shown in FIGS. 1-4. At the beginning of the bending process, the blank is bent at a fairly large radius which would be in the range of 5 to 10 material thicknesses or more. With further displacement of the punch 18, the blank is bent towards the stationary tool and until the distal edge 22 of the flange 20 engages the limiting edge 24 of the die 14. Further displacement of the punch 18 causes the edge of the blank to slide along the surface of the die 14. When the distal edge 22 of the flange 20 reaches the limiting edge 24 of the flange 20, an arc-shaped portion 40 is upset by the punch 18. Material from the arc-shaped portion flows across the forming side 38 of the die 14. Displacement of the material of the flange is restricted in the direction towards the distal edge 22 of the flange 20 by the limiting edge 24 of the die 14. This restriction of the displacement of the material of the flange 20 enhances the displacement of material towards the bending area. The area of the outer radius of the blank where the most stretching usually occurs is in contact with the punch 18. The clamping pad 16, limiting edge 24 and punch 18 crate a closed volume material flow into the base portion 26 with substantial overall compression applied to the surface of the panel.

Referring to FIG. 5, an alternative embodiment is shown wherein a die set 48 acts upon a panel 50. The die set 48 includes a die 52 that defines an inside corner 54 into which the panel 50 is to be formed. The portions of the die 52 adjacent the inside corner 54 may be referred to as an anvil surface. A base portion 56 of a flange 58 is initially formed in a separate forming operation or in a preceding forming operation in the die 52 to pre-form a flange 58. The flange 58 has an initial outside radius 60 that is greater than a minimum radius at which a fracture may occur from bending the flange using conventional forming techniques. The panel 50 is held in place by a clamping pad 62. A compressing tool 64 engages a distal edge 66 of the flange 58.

A forming tool 68 is moved into engagement with the base portion 56 of the flange 58. The forming tool 68 is moved independently, but in conjunction with the compressing tool 64. The forming tool 68 engages the panel 50 to form the flange 58 against the inside corner 54 of the die 52. The forming tool 68 forms the base portion 56 of the panel 50 to have a final outside radius 70 that corresponds to the radius of the inside corner 54. A final inside radius 72 is formed on the panel 50 and corresponds to the radius of a radius portion 74 of the forming tool 68. The flange 58 is formed by the forming tool 68 while a compressive force is exerted by the compression tool 64 against the distal edge 66 of the flange 58. The compressive force exerted by the compression tool 64 compresses the flange 58 against the inside corner 54 of the die 52.

The blank is pre-formed to have a generous radius substantially larger than the smallest bending radius where fracture may be initiated with conventional bending techniques This radius may be 2 to 4 times larger than the smallest outside radius that is possible with conventional bending techniques. Pre-bending can be performed with traditional bending processes. The second part of the process is performed with axial compression of the pre-bent flange into the sharp corner defining the sharp external radius of the bent panel. The panel is supported on both sides to prevent buckling of the portion of the blank being compressed into the sharp corner. The length of the flange at the end of the process is calibrated to avoid buckling. The forming tool 68 has an inside radius that corresponds to the final inside radius of the flange at the end of the process.

The displacement of the forming tool 68 can be calculated from the following considerations. The initial length of the flange including the curved area is:

L=1.57×R _(o) +h _(o)

Where R_(o) is the initial radius of the pre-bent sheet; h_(o) is the length of the flange initially extending beyond the curved area. The initial radius of curvature R_(o) is symmetrically transformed into a smaller radius R generating a flat area on the horizontal area of the flange and also an additional flat area on the vertical portion of the flange. The coordinates of the point where where the compression force is applied at the beginning of the process can be calculated according to the following formula:

h=R _(o) +h _(o)

Assuming that the initial radius is equal R_(o)=5t,

h=5 t+h _(o)

Assuming that the thickness of the sheet being bent, is not changing, and that the material flow is generally within the plane, the coordinate of the point at which the compressive force is applied can be calculated according to the following formula:

1.57R+(R _(o) −R)×2−X+h _(o)=1.57 R _(o) +h _(o),

Where X is the displacement of the punch providing compression of the blank in the in-plane direction, and R is the current radius of curvature of the sheet metal blank.

Then,

X=(2−1.57) (R _(o) −R)

The displacement is calculated below for several different cases where:

R _(o)=5t, R=4t X=0.43 (5t−4t)=0.43t

R _(o)=5t, R=3t X=0.43 (5t−3t)=0.86t

R _(o)=5t, R=2t X=0.43 (5t−2t)=1.29t

R _(o)=5t, R=1t X=0.43 (5t−t)=1.72t

The displacement of the second tool X₂ should be:

X₂=R_(o)−R

R=4t, X₂=t

R=3t, X₂=2t

R=2t, X₂=3t

R=t, X₂=4t

From this perspective, the forming tool 68 should have a higher velocity than the compression tool 64.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the disclosed apparatus and method. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure as claimed. The features of various implementing embodiments may be combined to form further embodiments of the disclosed concepts. 

What is claimed is:
 1. A method of forming a sharp bend on a panel comprising: pre-bending the panel to form a flange having a radius at a base portion of the flange between the flange and a body portion of the panel; clamping the flange between a distal end of the flange and the base portion with a clearance being defined between the flange and an anvil surface; and compressing the flange against the anvil surface.
 2. The method of claim 1 further comprising clamping the body portion of the panel between a clamping pad and a die while a punch is reciprocally moved to perform the pre-bending step and then performs the compressing step.
 3. The method of claim 2 wherein the die has a limiting edge that is engaged by the distal end of the flange during the compressing step.
 4. The method of claim 3 wherein the flange is formed into engagement with the anvil surface of the die by the punch, and wherein the distal end of the flange slides along the anvil surface until the distal end engages the limiting edge.
 5. The method of claim 1 wherein the clearance defined between the flange and an anvil surface is defined by an arc-shaped portion of the flange adjacent to the base portion of the flange and wherein the method further comprises compressing the arc-shaped portion against the anvil surface causing the arch-shaped portion to flow toward the base portion of the flange.
 6. The method of claim 1 further comprising: clamping the body portion of the panel between a clamping pad and a die; compressing the distal end of the flange between a compressing tool and the die; and forming the base portion of the flange into a corner of the die with a forming tool while the flange is compressed by the compressing tool.
 7. The method of claim 6 wherein the clearance is defined between the flange and the corner of the die and is defined by an arc-shaped portion of the flange at the base portion of the flange, and wherein the method further comprises causing the base portion of the flange to flow into the corner of the die.
 8. A tool for forming a flange on a panel comprising: a die having a limiting edge engaging a distal end of the flange; a clamping pad for clamping the panel against the die; and a punch initially engages the panel to form the flange with a radius at a base portion of the flange, and wherein the punch forms the flange against an anvil surface of the die while the flange is compressed between the clamping pad and the limiting edge to reduce the radius of the flange.
 9. The tool of claim 8 further comprising: an outside corner provided on the die, wherein the punch engages the panel to initially form the flange around the outside corner, and wherein the clamping pad clamps the panel on one side of the outside corner.
 10. The tool of claim 8 wherein the base portion of the flange has an arc-shaped portion that defines an arc-shaped clearance relative to the anvil surface, and wherein the punch forms the arc-shaped portion of the flange into the base portion of the flange.
 11. A tool for forming a flange on a panel comprising: a die having an inside corner that receives a base portion of the flange in a spaced relationship, wherein the base portion of the flange has an initial radius; a clamping pad for clamping the panel against the die; a compressing tool engaging a distal edge of the flange; and a forming tool that forms the base portion of the flange into the inside corner while the flange is compressed by the compressing tool to form a final radius that is smaller than the initial radius.
 12. The tool of claim 11 wherein the inside corner has a radius that corresponds to a final outside radius of the flange, and wherein the forming tool has a radius that corresponds to a final inside radius of the forming tool.
 13. The tool of claim 11 wherein the compressing tool moves with the forming tool to compress the flange while the forming tool forms the final radius.
 14. The tool of claim 11 wherein the flange is pre-formed in a forming die to a radius that is greater than a minimum radius at which a fracture may occur from bending the flange. 